This project investigates biobehavioral development through comparative longitudinal study of nonhuman primates, with special emphasis on characterizing individual differences among rhesus monkeys in their behavioral and physiological responses to mild environmental challenges and on determining the long-term developmental consequences for these individuals in different physical and social environments. During FY90 continued longitudinal study of genetic and environmental factors influencing response to challenge yielded findings that both broadened the scope of the phenomena and provided insight as to underlying mechanisms. A new technique for direct intraventricular peptide infusion permitted the assessment of changes in behavior and pituitary-adrenocortical activity in socially living rhesus monkeys following different doses of CRF and CRF anatagonists. Investigations of adolescent rhesus monkeys living in wild settings validated the relationship between individual differences in response to challenge and levels of adrenocortical activity and monoamine turnover previously reported for laboratory-housed adolescent monkeys. A second ongoing field study further related patterns of biobehavioral response to challenge to differential patterns of adolescent male emigration and mortality risk. Completion and staffing of a neonatal nursery permitted the initiation of a new set of studies focusing on rhesus monkey neonatal psychophysiological, neurochemical, attentional, and behavioral predictors of differential response to challenge later in life. Completion of new facilities for housing New World primates made it possible to establish a self-sufficient colony of Cebus apella and to begin parallel studies of factors influencing biobehavioral response to challenge in this primate species.